1. Field of the Invention
This invention relates to apparatus for flanging sheet metal blanks comprising a machine frame and a holding-down mechanism, which is adapted to hold down a sheet metal blank on a backing structure and comprises a holding-down shoe, which is composed of a plurality of segments arranged in a straight row, and a holding-down ram, which is guided in said frame and operable to force said holding-down shoe onto said blank on said backing structure so that said segments define on said blank a bend line about which said blank is adapted to be flanged, wherein said segments comprise mid-segment means, which are mounted to be displaceable transversely to said row, and side segments, which are disposed on opposite sides of said mid-segment means and are displaceable along said row.
2. Description of the Prior Art
When such apparatus is to be used to form sheet metal blanks with edge flanges, the holding-down shoe must be adjustable in length to the length of the edge flange that is to be formed. From EP A-0 105 091 it is known that this can be accomplished in that the holding-down shoe is composed of a plurality of segments, which are arranged in a row that is parallel to the bend line and can be adjusted along said row. In that case the selection of the number of segments used at a time will permit a coarse adaptation of the length of the holding-down shoe to the length of the edge flange that is to be formed. A fine adjustment is permitted in that a set of narrow segments are provided in the middle portion of the holding-down shoe and each of said narrow segments is pivoted to the holding-down ram on an axis which is parallel to said row so that said narrow segments can selectively be swung in and out for an adjustment of the length of the holding-down shoe in steps which are equal to the length of the narrow segments. But such an arrangement does not permit of a sufficiently fine adjustment because when the blank is to be formed with inturned edge flanges having two or more bends the outermost segments of the holding-down shoe must be displaced inwardly in order to shorten the holding-down shoe so that the latter can be lifted from the workpiece between the inturned edge flanges. Such an inward displacement of the segments of the holding-down shoe is permitted in that a mid-segment is provided at each end of the set of narrow segments and can be pulled out of the row of segments by a movement that is transverse to the row and parallel to the surface of the sheet metal blank and the side segments which are disposed on the outside of said mid-segments can then be displaced inwardly to an extent which is equal to the length of the mid-segment.
That known apparatus involves a relatively high structural expenditure because three kinds of segments are required and the segments of each kind must be separately actuated. Besides, the space required for the transverse displacement of the mid-segments in a direction which is parallel to the surface of the sheet metal blank within the contour of said blank will define a lower limit for the size of the blank. Moreover, the replacement of individual segments is expensive and involves a relatively large downtime of the apparatus.